This invention relates to selected polysaccharide laminating or bonding adhesives having high bond strength and low bleed properties and which are useful in multi-ply, light weight paper products.
The use of paper, particularly light weight paper in articles such as tissue or towels is well known. In order to add and improve properties such as absorbency, bulk, smoothness, drape and hand (feel), these products are often fabricated in multi layers, i.e., multi-ply articles. In the end use of these multi-ply articles and in converting steps such as slitting, rewinding, folding and printing, it is desirable that the plies remain in register and not slip in relation to each other. This is accomplished by bonding the plies and is conventionally done by mechanical means such as embossing or through the use of an adhesive.
Mechanical means of bonding multi-ply articles by embossing through all plies in bands near the article""s edge or in partial coverage patterns distributed across the surface is often used. However, such means generally provide low bond strength.
Adhesives have also been used to provide bonds for multi-ply articles. They are generally applied at low solids (typically less than 10% by weight) and depending on the articles being made, either by non-contact methods such as spraying or by contact methods using transfer rolls. While various adhesive materials such as cellulose, polyvinyl alcohol and starch have been used in these applications, depending on the particular composition they often have one or more problems such as bleed, low strength or application difficulties (e.g., slow fabrication speeds and slow bond development) due to low solids content.
Illustrative preparation of multi-ply paper products is disclosed in WO 96/24485 published on Aug. 15, 1996 to T. Rydell et al. This patent publication discloses the use of various adhesives such as silicates, glue, starch adhesives, cellulose and polyvinyl alcohol and utilizes the high bleed, strike through characteristics of these materials in preparing mutli-ply paper laminates.
Another disclosure of preparing multi-ply paper products is found in U.S. Pat. No. 3,673,060 issued Jun. 27, 1972 to J. Murphy et al. This patent discloses the combination of embossing means plus the use of an adhesive such as polyvinyl alcohol to prepare laminated napkins.
Despite the well known use of different adhesives in preparing multi-ply paper products as illustrated above, there is still the need for new adhesive materials which can provide high strength, low bleed characteristics and are readily applicable in different application techniques.
It has now been found that selected high solids, polysaccharide based aqueous adhesives that provide high bond strength and low bleed properties and also are repulpable, have other properties that makes them particularly useful in the preparation of multi-ply light weight paper products.
More particularly, this invention involves an aqueous, high-solids, polysaccharide based adhesive composition comprising a continuous aqueous phase of from about 15 to 50% by weight of starch/dextrin material or blend thereof dispersed in water, the continuous phase starch/dextrin or blend having:
a) a zero-shear viscosity of greater than 0.07 Paxc2x7s when measured at 26% (+/xe2x88x921%) solids, 120xc2x0 F. (49xc2x0 C.) and a shear rate of 0.1sxe2x88x921,
b) a shear thinning index of 0.9 to 1.1 and
c) a bleed time of greater than 15 seconds.
This invention further involves an adhesive composition comprising the continuous phase starch/dextrin dispersed aqueous component described above, as well as a second component comprising a water swellable, insoluble polysaccharide particulate phase, the adhesive composition having a bleed time of greater than 70 seconds when measured at 19% (+/xe2x88x921%) solids at 120xc2x0 F. (49xc2x0 C.).
This invention further involves a continuous phase polysaccharide adhesive composition, particularly for contact applications, comprising a starch/dextrin material wherein the continuous phase has a zero-shear viscosity of greater than 0.05 Paxc2x7s measured at 26% (+/xe2x88x921%) solids, 120xc2x0 F. (49xc2x0 C.) and a shear rate of 0.1sxe2x88x921 and a bleed time of greater than 7 seconds.
This invention is directed to adhesives used in the fabrication of multi-ply light weight paper products. Such products include tissue and towels such as facial tissue, toilet or sanitary tissue, napkins, paper towels as well as embossed towels. The term xe2x80x9cpaperxe2x80x9d as used herein refers to products made from cellulosic fibers in a wet end or wet laid process. This includes pulp or paper derived from wood pulp, sulfate or sulfite pulp. The light weight paper products as described herein are typically made using paper of less than about 30 lb/3300 ft2.
The methods of applying adhesive for use in the manufacture of multi-ply paper products are well known. Typically, they involve a non-contact method where the adhesive is applied by spraying or atomization or a contact method where the adhesive is transferred from a roll in contact with the inner surface of a web. The spray, non-contact method is particularly useful in the preparation of tissue products while the contact transfer method is predominantly used in the preparation of paper towels where embossing is often employed. The adhesive that is used in preparing multi-ply paper products will depend on the particular application and the conditions used. The polysaccharide based adhesive composition of this invention can be used or varied and adapted so that it can be used in either non-contact or contact applications as further described herein.
The base polysaccharide material used in the continuous phase of the adhesive of this invention is characterized by specific rheological properties. The polysaccharide may comprise a starch or dextrin or a blend of starch and dextrin. The polysaccharide may comprise a starch derived from any plant source including corn, potato, wheat, rice, tapioca, waxy maize, waxy rice, sago, sorgum, high amylose starch such as high amylose corn, etc. Also include are the conversion product derived from any of the former bases including, for example, dextrins prepared by the hydrolytic action of acid and/or heat such as maltodextrin and pyrodextrin; oxidized starches prepared by treatment with oxidants such as sodium hypochlorite; fluidity or thin-boiling starches prepared by enzyme conversion or mild acid hydrolyses; derivatized starches such as starch ethers and esters and physically modified starches. The polysaccharide component may also include water soluble gums to help the composition satisfy desired properties such as solids content, Theological and bleed properties.
The aqueous dispersed continuous phase must satisfy certain rheological properties including a zero-shear viscosity and shear-thinning index as well as providing selected bleed requirements. Typical polymeric solutions, such as the aqueous dispersed-continuous phase types described herein, can show a viscosity that is dependent on rate, i.e., the resistance to deformation (viscosity) is a function of the deformation rate (shear rate). It is well known in the rheology literature that the viscosity of these materials approach a constant value at low shear rates called xe2x80x9czero-shear viscosityxe2x80x9d. The test procedure for determining the zero-shear viscosity is described below. The starch-dextrin dispersed phase as used in this invention will have a zero-shear viscosity of greater than 0.07Paxc2x7s when measured at 26% (+/xe2x88x921%) solids, 120xc2x0 F. (49xc2x0 C.) and a shear rate of 0.1sxe2x88x921. Preferably the zero-shear viscosity will be greater than 0.1 Paxc2x7s.
The other rheological property for the aqueous dispersed, continuous phase component is known as the xe2x80x9cshear-thinning indexxe2x80x9d (nxe2x80x2). The shear-thinning index is a measure of the reduction in viscosity across a defined deformation rate change. The test procedure for determining the shear-thinning index is also described below. The aqueous dispersed continuous phase as used in this invention will have a shear-thinning index of between 0.90 and 1.1, preferably between 0.95 and 1.1.
The aqueous dispersed, continuous phase of this invention must also satisfy certain bleed requirements in addition to the reheological properties. Bleed is defined as a measure of the tendency of the adhesive to penetrate into the substrate. Materials having a low bleed tendency, or high bleed time are expected to provide suitable laminating adhesive performance. Materials with a high bleed tendency, or low bleed time, are known not to perform and are characterized by xe2x80x9cstrike throughxe2x80x9d or transfer of the adhesive through the bonded substrate layers. A detailed description of the bleed test used is described below. The adhesive composition of this invention containing the aqueous dispersed continuous phase will have a bleed time of greater than 15 seconds and preferably greater than 50 seconds.
The aqueous dispersed continuous polysaccharide component as described herein can be prepared for use as an adhesive by cooking and dispersing the polysaccharide in water at a solids of about 15 to 50%. The polysaccharide may be cooked using any of the known techniques including atmospheric cooking, jet cooking, steam injection cooking, high shear cooking, retorting or autoclaving. Typical cooking conditions can range from a temperature of at least the gelatinization temperature of the material and this can be from about 70 to 200xc2x0 C. or higher depending on the conditions and type of cooking being utilized. Additionally, the polysaccharide component can be physically processed to render it pregelatinized by any of several methods known in the art, such as drum drying and extrusion etc., prior to the addition of water.
As described above, the particular adhesive used in preparing multi-ply products will depend to some extent on the type of application. While the composition having a dispersed or continuous phase, as described above, is useful in non-contact application, it has been found that in spray-type applications, a variation in the composition will provide improved properties. This variation involves the addition of a second component comprising a water swellable, insoluble polysaccharide particulate phase. This second component helps to improve sprayability by providing better atomization and control of atomization and also helps to reduce bleed. The particulate polysaccharide second component of the adhesive composition may comprise any polysaccharide that provides swellable, discrete particles that are not soluble. This, for example, could be a starch which is not fully cooked out and contains swollen granules or discrete particles.
More particularly, the water swellable, insoluble polysaccharide particulate that comprises the second component of the adhesive composition will be a crosslinked starch, thermally inhibited starch or crosslinked (chemically or natural) gum that when cooked swells up and retains a particulate structure and doesn""t disintegrate during processing and use.
The crosslinked starch used in this invention are those known in art and may include starch which is treated with a number of multi-functional crosslinking agents such as disclosed in xe2x80x9cStarch Derivatives: Production and usesxe2x80x9d by M. Rutenberg and D. Solarek, Starch: Chemistry and Technology, Chapter X, pp. 324-332, 1984. Such crosslinking agents include bifunctional etherifying and/or esterfying agents such as epichlorohydrin, bis-chloroethyl ether, dibasic organic acids, phosphorus oxychloride, trimetaphosphate (i.e., the alkali and alkaline earth metal salts), linear mixed anhydrides of acetic and di- or tribasic carboxylic acids. Another useful crosslinking agent is sodium hypochlorite, which when used in the proper amount and under proper pH conditions (11 or more) provides crosslinked starches. Particularly useful crosslinking agents are epichlorohydrin, phosphorus oxychloride, adipic-acetic anhydrides and sodium trimetaphosphate. The amount of crosslinking is not critical and will generally be a small amount that is sufficient to allow the cooked material to swell up and not disintegrate. The crosslinked starch may also be further modified with other reagents to provide cationic, anionic or amphoteric groups. One particularly useful group is cationic modification with tertiary amino or quaternary ammonium ether groups.
Thermally inhibited starches where the starch is heat treated in a selected manner may be used instead of or in addition to the crosslinked starches. Thermally inhibited starches are known and have been described in U.S. Pat. 5,725,676 issued Mar. 10, 1998 to C. Chiu et al., which patent is incorporated herein by reference.
The amount of water swellable, insoluble polysaccharide particulate material used as the second component of the adhesive composition will vary from about 1.5 to 12.5% by weight based on the total weight of aqueous composition. In this formulation, the continuous or dispersed polysaccharide phase will comprise from about 13.5 to 48.5% by weight based on the total weight of the aqueous composition.
The polysaccharide particulate component needs to be suspended in water. Except for cold water dispersable material, this will involve the same known cooking techniques such as atmospheric or jet cooking as described above. This cooking may take place separately for each component or more preferably will involve one single cooking operation where all components and ingredients are combined.
When using a contact application procedure such as one involving transfer rolls, the adhesive that is preferably used will be one involving the continuous aqueous polysaccharide (starch/dextrin) phase. This continuous phase will have varied properties from the one described previously and will have a zero-shear viscosity of greater than 0.05 Paxc2x7s, preferably greater than 0.2 Paxc2x7s and preferably less than 20 Paxc2x7s measured at 26% (+/xe2x88x921%) solids, 120xc2x0 F. (49xc2x0 C.) and a shear rate of 0.1sxe2x88x921. Also, this adhesive with continuous polysaccharide phase for use in contact applications will have a bleed time of greater than 7 seconds and preferably greater than 10 seconds.
In addition to the continuous phase component and the particulate component when used, minor amounts of conventional additive components may optionally be added to the adhesive formulations of this invention. Such additives and ingredients include defoamers, preservatives or biocides, tackifiers, detackifiers, wetting agents, release agents, dyes, etc. Typically, these additives will comprise from 0 to 5% by weight based on the total weight of the composition.
The following test procedures were used to determine the rheological and bleed properties for the adhesives of this invention.
Rheology Test
The rheology tests for measuring zero-shear viscosity and the shear-thinning index are carried out on a Rheometrics Fluids Spectrometer II (Rheometrics Scientific, Piscataway, N.J.). Measurements were made using the couette geometry in all cases. The testing procedure utilized is based on standard techniques in the art and is described in detail by C. W. Macosko (Rheology Principles, Measurement and Applications, Chapters 1, 2 and 5, VCH, NY, 1994).
To measure the zero-shear viscosity, a sample of the selected aqueous dispersed phase component (starch dextrin or blend) was fully dispersed at 26% (+/xe2x88x921%) solids by weight and placed in the couette rheometer cell. The instrument was set in steady-state mode at 120xc2x0 F. (49xc2x0 C.) and rate sweep was performed from 0.1 to 100sxe2x88x921 with 5 points per decade of shear-rate. Each value of shear-rate was applied for 60 seconds, with no torque reading taken during the initial 30 seconds. By the end of this period, any time effects that were likely to occur had been reduced to levels that would not influence the viscosity measurement. Torque readings obtained during the final 30 seconds at each shear rate were converted to viscosity values, based on the geometry of the fixture used and the torsion constant of the instrument.
The resultant viscosity-shear rate data was analyzed for the zero-shear viscosity, taken as the viscosity at a shear-rate of 0.1sxe2x88x921.
The shear-thinning index was determined by first calculating the slope of the viscosity versus shear-rate plot between shear rates of 0.1 and 100sxe2x88x921 and then using the formula:
nxe2x80x2=1+slope
where nxe2x80x2 is the shear-thinning index, and the slope is determined as described above.
Bleed Test
Bleed tests were performed on the test section of a Rame-Hart NRL C. A. Goniometer (model no. 100-00 115). This device allowed a magnified and definable side view of a fluid drop placed on a substrate. The test liquid for the continuous phase component was measured at 26% (+/xe2x88x921%) and 120xc2x0 F. (49xc2x0 C.). The bleed test for the combined continuous and particulate (total composition) was measured at 19% (+/xe2x88x921%) and 120xc2x0 F. (49xc2x0 C.).
The bleed test involved using the unembossed portion of one web of Moresoft(trademark) luxury dinner napkin item #30100, manufactured by Morcon INC, Cambridge, N.Y., 12816, basis weight: 10.6 lbs/3300 ft2, placed over a strip of blotter paper. This substrate conglomerate was placed on the test section of the goniometer under slight tension such that the napkin stock was in intimate contact with the blotter paper without any wrinkles, air pockets or other discontinuities. The height of the test section was adjusted such that the top of the napkin stock coincided with the bottom of the graduated reticle on the eyepiece. A 50 xcexcl drop of test liquid from an Eppendorf 100 xcexcl micropipette was placed on the napkin stock such that it was visible in the eyepiece and the time required for the drop to reduce to one-quarter its original height measured. The graduated reticle on the eyepiece served as the measurement device for the change in height. The bleed test number is an average of at least five measurements for each sample being investigated. The napkin stock and blotter was moved between tests to ensure no interaction between the measured drop and previous samples.